Biological treatment processes for the removal of biological nutrients such as biological oxygen demand (“BOD”), nitrates and phosphates are well known. A typical biological treatment process is an activated sludge process in which the wastewater is aerated and agitated with an activated sludge and then purged of a variety of microorganisms. Often this aerobic stage is combined with an anaerobic stage, i.e., a stage operated in the absence of induced oxygen, either soluble or derived from nitrites or nitrates (NOx) and an anoxic stage, i.e., where oxygen is absent but nitrites or nitrates are present. Phosphorus removal is accomplished by the presence of phosphorus-accumulating organisms (“PAOs”) in the anaerobic stage which release phosphorus into the wastewater as part of the process of accumulating organic matter (i.e., volatile fatty acids) used for cell growth. In a downstream aerobic or anoxic zone, the organisms metabolize the accumulated organic matter and accumulate the released phosphorus into cells as part of the growth process. A number of prior art patents disclose multi-zoned bioreactors with some recycling of flows between the various zones to maintain concentrations of useful microorganisms and to improve biological nutrient removal. For example, Daigger, U.S. Pat. No. 6,517,723, the contents of which are incorporated herein by reference. Other examples include Daigger, U.S. Pat. No. 5,480,548; Hawkins, U.S. Pat. No. 5,601,719; Marsman, U.S. Pat. No. 5,342,522; Strohmeier, U.S. Pat. No. 5,798,044; Hong, U.S. Pat. No. 5,650,069; Timpany, U.S. Pat. No. 5,354,471; Wittmann, U.S. Pat. No. 4,961,854; Nicol, U.S. Pat. No. 4,787,978; and Yang, U.S. Pat. No. 5,942,108.
In certain circumstances, very low concentrations of phosphates in effluents are required and in such circumstances chemical precipitation is used for phosphate or phosphorous removal. In chemical precipitation methods, soluble salts, such as ferrous/ferric chloride or aluminum sulfate, are added to the wastewater to form insoluble phosphate metal salts. The insoluble phosphate metal salts are then gravity separated or filtered from the wastewater to yield an effluent with low concentrations of total phosphate (“TP”). Low levels of TP are defined herein to be in a range of less than 0.25 mg/L.
As disclosed in Husain, U.S. Pat. No. 6,406,629, a type of biological treatment known as a membrane bioreactor can be combined with phosphate precipitation techniques. Husain sets forth an example of addition of phosphate precipitating chemicals to an aerobic tank connected to a membrane filter. This combination is criticized, however, because the presence of metallic precipitates increases the rate of membrane fouling or forces the operator to operate the system at an inefficient long sludge retention time. Another disadvantage of a combined system as described is if the system includes recycle of activated sludge, which is typical in biological processes, removal of phosphorous in excess of the stoichoimetric amount of phosphorous required to support growth of the activated sludge in the biological treatment process can degrade the efficiently of the biological treatment process' removal of other nutrients.
Husain describes one attempted solution to the problems discussed above. Husain provides side stream processes operating in parallel to a conventional multistage activated sludge biological treatment process to remove excess phosphorous. In a first side stream process, a liquid lean in solids but containing phosphates is extracted from anaerobic mixed liquor from an anaerobic stage of the activated sludge process. Phosphates are precipitated from that mixed liquor to produce a phosphorus lean liquid which leaves the process as effluent or is returned to an anoxic or aerobic zone. In an alternate side stream process, an aerobic mixed liquor is removed to a reaction zone and treated to form a liquid rich in insoluble phosphates. The liquid rich in insoluble phosphates is treated in a hydro cyclone to separate out insoluble phosphates and create a liquid lean in insoluble phosphates. The liquid lean in insoluble phosphates is returned to the anoxic zone. While the solution proposed in Husain may provide for effective phosphorous removal, it requires additional processes which increase treatment costs along with the space required to perform the treatment process. In addition, the first side stream process may result in a phosphorous deficiency in biological process stages downstream from the anaerobic zone.
The present invention is directed toward overcoming one or more of the problems discussed above.